1. Field of the Invention
The present invention relates to a device for driving a stepping motor which is capable of driving the motor with high efficiency by improving the leading edge characteristics of input pulses supplied to the motor.
2. Description of the Prior Art
It is well known that a stepping motor which is also referred to as a pulse motor, in general, is a motor in which a plurality of phase coils are excited or energized with input pulses by changing them over one by one, thereby rotating its rotor by angles corresponding to the number of input pulses. Such a motor has been widely utilized for electrically controlling mechanical rotation and action of a machine since control of the stepping motor can easily be effected in addition to simple construction and low price.
As to stepping motors there are three phase stepping motors, four phase stepping motors and five phase stepping motors. The driving circuit of the stepping motor utilizes the system of 1 phase excitation and 1-2 phase excitation as well as micro-stepping drive for precise control. Moreover, the leading edge characteristics of input pulses affect the driving characteristic of the stepping motor since as described above the pulses continuously supplied therein one by one with a certain phase difference are changed over thereby obtaining the driving power. When the input pulses are changed over to distribute them to respective phase coils the leading edge of the pulses does not become steep because of a lagging current due to an inductance of a motor driving phase coil resulting in a threshold of rotational speed and an unstable rotational speed.
One embodiment of the conventional stepping motor driving device is shown in FIG. 1 and its operation is described with reference to FIG. 2.
In FIG. 1 reference PM is a drive phase section of a stepping motor (not shown). The drive phase section comprises four phase coils A, B, C, and D and one end portion of respective coils is connected in common and to a power supply terminal P.sub.3 through a resistor R.sub.1. The terminal P.sub.3 is connected to a power supply source (not shown) which supplies a supply voltage Vcc. The other end portions of respective coils A, B, C and D are connected to a grounded point through collector-emitter paths of switching transistors TR.sub.1, TR.sub.2, TR.sub.3 and TR.sub.4, respectively. That is, the transistor TR.sub.1 is connected to the phase coil A, the transistor TR.sub.2 to the phase coil B, the transistor TR.sub.3 to the phase coil C and the transistor TR.sub.4 to the phase coil D. The base electrodes of respective transistors TR.sub.1 to TR.sub.4 are connected to the output terminals of a control pulse distributor circuit CPD through resistors R.sub.2, respectively. The pulse distributor circuit CPD has a drive pulse input terminal P.sub.1 and an UP/DOWN signal input terminal P.sub.2 and generates control signals CP.sub.1, CP.sub.2, CP.sub.3 and CP.sub.4 which control the ON and OFF states of the transistors TR.sub.1, TR.sub.2, TR.sub.3 and TR.sub.4 in regular order. In such a conventional stepping motor the supply voltage Vcc is 10 to 20 V and the resistor R.sub.1 has a value of 5 to 10.OMEGA..
When the supply voltage Vcc is applied to the terminal P.sub.3 and the control pulse distributor circuit CPD distributes control pulses CP.sub.1 to CP.sub.4 to the switching transistors TR.sub.1 to TR.sub.4, respectively, in regular order, phase currents i.sub.A, i.sub.B, i.sub.C and i.sub.D as shown in FIG. 2 flow in the phase coils A to D, respectively, resulting in a generation of magnetic flux .phi..sub.A, .phi..sub.B, .phi..sub.C or .phi..sub.D, thereby driving the stepping motor. As is seen from FIG. 2 the phase currents i.sub.A to i.sub.D rise with slow curve and reach a stationary state. The time for reaching such a state is about 0.5 to 1 msec. In this case the generated voltage is 10 to 20 V as a kick voltage at a leading edge instant of the current and is 100 to 120 V as a trailing edge pulse at a trailing edge instant of the current. In order to improve such a voltage characteristic in the conventional circuit as it is, therefore, it is necessary to make the supply voltage Vcc equal to or more than 100 V and the value of resistor R.sub.1 equal to or more than 20.OMEGA., so that the resistor R.sub.1 with such a high resistance value consumes a high power and it can not be practically used. In the stepping motor driven by a battery which is carried on a vehicle, particularly, the driving device with such a resistor can not be utilized. In order to drive the stepping motor, therefore, it is desirable to improve the leading edge characteristics of input pulses.